Pre-press color-proofing is a procedure that is used by the printing industry for creating representative images of printed material without the high cost and time that is required to actually produce printing plates and set up a high-speed, high volume, printing press to produce an example of an intended image. These representative images may require several corrections and be reproduced several times to satisfy the customer. Pre-press color-proofing saves time and money getting to an acceptable finished product.
An example of a commercially available image processing apparatus is shown in commonly assigned U.S. Pat. No. 5,268,708 and has half-tone color proofing capabilities. This image processing apparatus is arranged to form an intended image on a sheet of thermal print media in which dye from a sheet of dye donor material is transferred to the thermal print media by applying thermal energy to the dye donor material. The image processing apparatus is comprised generally of a material supply assembly or carousel, a lathe bed scanning subsystem (which includes a lathe bed scanning frame, translation drive, translation stage member, printhead, and vacuum imaging drum), and the thermal print media and dye donor material exit transports.
The operation of the image processing apparatus comprises metering a length of the thermal print media, in roll form, from the material assembly or carousel. The thermal print media is measured and cut into sheets of required length, transported to the vacuum imaging drum, registered and wrapped around and secured to the vacuum imaging drum. A length of dye donor material in roll form is metered out of the material supply assembly measured and cut into sheets of required length. The dye donor material is transported to and wrapped around the vacuum imaging drum, superposed and in registration with the thermal print media.
The thermal print media and the dye donor material are held on the spinning vacuum imaging drum by a vacuum and applied through holes in the surface of the drum while it is rotated past the printhead. The translation drive moves the printhead and translation stage member axially along the vacuum imaging drum in coordinated motion with the rotating vacuum imaging drum to produce the intended image on the thermal print media.
After the intended image has been written on the thermal print media, the dye donor material is removed from the vacuum imaging drum without disturbing the thermal print media beneath it. The dye donor material is transported out of the image processing apparatus by the dye donor material exit transport. Additional sheets of dye donor material, each a different color, are sequentially superimposed with the thermal print media on the vacuum imaging drum and imaged onto the thermal print media as described above, until the intended image is completed. The completed image on the thermal print media is unloaded from the vacuum imaging drum and transported to an external holding tray on the image processing apparatus by the exit transport.
The vacuum imaging drum is cylindrical in shape and includes a hollow interior portion. A plurality of holes extends through a surface of the drum applying a vacuum from the interior of the vacuum imaging drum, which maintains the position of the thermal print media and dye donor material as the vacuum imaging drum rotates.
The ends of the vacuum imaging drum are enclosed by cylindrical plates, each containing a centrally disposed spindle. The spindles extend through support bearings and are attached to the lathe bed scanning frame. The drive end spindle extends through the support bearing and is stepped down to receive a DC drive motor armature. The opposite spindle is provided with a central vacuum opening in alignment with a vacuum fitting with an external flange that is rigidly mounted to the lathe bed scanning frame. The vacuum fitting has an extension which is closely spaced with the vacuum spindle forming a small clearance. This configuration provides a slight vacuum leak between the outer diameter of the vacuum fitting and the inner diameter of the opening of the vacuum spindle. This assures that no contact exists between the vacuum fitting and the vacuum imaging drum which might impart uneven movement to the vacuum imaging drum during its rotation.
The opposite end of the vacuum fitting is connected to a high-volume vacuum blower which is capable of producing 50-60 inches of water (93.5-112.2 mm of mercury) at an air flow volume of 60-70 cfm (28.368-33.096 liters/sec). The vacuum required varies during the loading, scanning, and unloading of the thermal print media and the dye donor materials. With no media loaded on the vacuum imaging drum, the internal vacuum level of the vacuum imaging drum is approximately 10-15 inches of water (18.7-28.05 mm of mercury). With the thermal print media loaded on the vacuum imaging drum, the internal vacuum level of the vacuum imaging drum is approximately 20-25 inches of water (37.4-46.75 mm of mercury). This level is required when a dye donor material is removed, otherwise the thermal print media may move and color-to-color registration will not be maintained as dye donor material sheets are changed. With both the thermal print media and dye donor material completely loaded on the vacuum imaging drum, the internal vacuum level of the vacuum imaging drum is approximately 50-60 inches of water (93.5-112.2 mm of mercury).
The outer surface of the vacuum imaging drum is provided with an axially extending flat, which extends approximately 8.degree. around the vacuum imaging drum circumference. The vacuum imaging drum is also provided with a circumferential recess which extends circumferentially from one side of the axially extending flat circumferentially around the vacuum imaging drum to the other side of the axially extending flat, and from approximately one inch (24.5 mm) from one end to approximately one inch (25.4 mm) from the other end of the vacuum imaging drum. The thermal print media, when mounted on the vacuum imaging drum, is seated in the circumferential recess. The circumferential recess has a depth substantially equal to the thermal print media thickness, approximately 0.004 inches (0.102 mm).
The purpose of the circumferential recess on the vacuum imaging drum surface is to eliminate any creases in the dye donor material as it is drawn down over the thermal print media during loading. This assures that no folds or creases will be generated in the dye donor material which could extend into the image area which would adversely affect the intended image. The circumferential recess also substantially eliminates the entrapment of air along the edge of the thermal print media where it is difficult for the vacuum holes in the vacuum imaging drum surface to assure the removal of the entrapped air. Any residual air between the thermal print media and the dye donor material can also adversely affect the intended image.
The purpose of the vacuum imaging drum axially extending flat assures that the leading and trailing ends of the dye donor material are protected from the effects of air drag during high speed rotation of the vacuum imaging drum during imaging process. Without the axially extending flat, the air drag tends to lift the leading or trailing edge of the dye donor material. The vacuum imaging drum axially extending flat also ensures that the leading and trailing ends of the dye donor material are recessed from the vacuum imaging drum periphery. This reduces the chance that the dye donor material contacting other parts of the image processing apparatus, such as the printhead, which may cause a jam, loss of the intended image, or catastrophic damage to the image processing apparatus.
The task of loading and unloading the dye donor material on the vacuum imaging drum requires precise positioning of thermal print media and the dye donor materials. The lead edge positioning of dye donor material must be accurately controlled during this process. The existing image processing apparatus design employs a multi-chambered vacuum imaging drum for such lead-edge control. One chamber applies vacuum to hold the leading edge of the dye donor material. Another chamber, separately valved, controls vacuum which holds the trailing edge of the thermal print media to the vacuum imaging drum. With this arrangement, loading a sheet of thermal print media and dye donor material requires that the image processing apparatus feed the lead edge of the thermal print media and dye donor material into position just past the vacuum ports controlled by the respective valved chamber. As vacuum is applied, the leading edge of the a dye donor material is pulled against the vacuum imaging drum surface.
Unloading the dye donor material, or the thermal print media, requires removal of vacuum from these same chambers so that an edge of the thermal print media, or the dye donor material, is freed and projects out from the surface of the vacuum imaging drum. The image processing apparatus then positions an articulating skive into the path of the free edge to lift the edge and to feed the dye donor material to a waste bin or the thermal print media to an output tray.
Although the image processing apparatus described is satisfactory, there is room for improvement. The technology utilized in the above image processing apparatus does not allow for large format thermal print media and dye donor material. Also throughput, the number of intended images per hour, is limited by the vacuum imaging drum rotational speed. (The faster the vacuum imaging drum rotates, the faster the output of the intended image can be exposed onto the thermal print media, thus increasing the throughput of the image processing apparatus.) At high rotational speeds, in excess of 1000 RPM, increased air turbulence and centrifugal force can separate the thermal print media and dye donor materials from each other and from the vacuum imaging drum, thus limiting the rotational speed of the vacuum imaging drum.
One approach to solving the above problem is adding external clamping components to hold the thermal print media and dye donor material on the vacuum imaging drum. This, however, adds increased cost and introduces added mechanical complexity to the vacuum imaging drum design. This solution may also cause the vacuum imaging drum to go out of round as much as 80 microns (0.0032 inches), which would not allow the image processing apparatus to meet image quality specifications. (The image processing apparatus tolerance requirement for focus is approximately 10 microns or 0.004 inches.) Clamping the thermal print media and dye donor material would also introduce a clearance problem since the working distance of the printhead to the surface of the thermal print media loaded on the vacuum imaging drum is approximately 0.030 inches (0.762 mm).
Another way to prevent the increased air turbulence and centrifugal force from separating the thermal print media and dye donor material from the rotating vacuum imaging drum is to add more vacuum holes to the surface of the vacuum imaging drum, or enlarge the diameter of the vacuum holes. This, however, would require an increase in the vacuum level in the interior of the vacuum imaging drum. A higher vacuum will increase the cost of the blower that produces the vacuum, requiring a complex vacuum coupling, adding mechanical noise to the rotation of the vacuum imaging drum, and increase customer operating cost by increasing electrical consumption. In addition, there is a limit to how high the vacuum level can be without distorting the media, which would decrease the quality of the intended image.